Hitherto, as the element member of semiconductor devices, photosensitive devices for use in electrophotography, image input line sensors, image pickup devices, or other optical devices, there have been proposed a number of amorphous semiconductor films, for instance, an amorphous deposited film composed of an amorphous silicon material compensated with hydrogen atoms (H) or/and halogen atoms (X) such as fluorine atoms or chlorine atoms [hereinafter referred to as "a-Si(H,X)"]. Some of such films have been put to practical use.
Along with those amorphous semiconductor films, there have been proposed various methods for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave glow discharging to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. And there have been also proposed various apparatus for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition process by means of microwave glow discharging decomposition [hereinafter expressed by the abbreviation "MW-PCVD process"] also at industrial level.
One representative apparatus for practicing such MW-PCVD process is such that has a structure as shown in a schematic drawing of FIG. 3 (this apparatus will be hereinafter referred to as "MW-PCVD apparatus").
In FIG. 3, there are shown a substantially enclosed deposition chamber 301, a microwave introducing window 302 (or 302') made of a dielectric material such as alumina ceramics or quartz which is connected through a waveguide to a microwave power source (not shown), an exhaust pipe 303 extending through an exhaust valve 304 from a vacuum pump 305, a raw material gas feed pipe 306 having a plurality of gas liberation holes (not shown) which is extending through a flow rate control valve 307 from a gas reservoir (not shown), a plurality of cylindrical substrates 308 each being positioned on a rotatable cylindrical substrate holder having an electric heater 309 therein, and a drive motor 310 for rotating said cylindrical substrate holder. The deposition chamber 301 ordinarily has a cavity resonant structure so as to resonate with the frequency of the microwave power source. Film-formation in the apparatus shown in FIG. 3 is carried out, for example, in the following manner.
That is, the inside of the deposition chamber 301 is evacuated by opening the exhaust valve 307 of the exhaust pipe 306 to bring the inside to a desired vacuum. The heater 309 is actuated to uniformly heat the cylindrical substrate 308 to a desired temperature while rotating it by the drive motor 310 and it is kept at this temperature. Then, raw material gases, for instance, silane gas and hydrogen gas, etc. in the case of forming an amorphous silicon deposited film, are introduced into the deposition chamber 301 through the gas feed pipe 306 and its gas liberation holes.
At the same time, microwave having a frequency of more than 500 MHz, preferably of 2.45 GHz is generated by the microwave power source, which is successively introduced into the deposition chamber 301 through the microwave introducing window 302 (or/and 302'). The raw material gases thus introduced into the deposition chamber 301 are excited and dissociated by an energy of the microwave to generate neutral radical particles, ion particles, electrons and the like and to cause chemical reactions among them resulting in formation of an amorphous silicon deposited film on the surface of each of the cylindrical substrates 308.
However, with the conventional MW-PCVD apparatus having such constitution as described above, there are the following disadvantages: (i) it is necessary to put the inside of the deposition chamber to the atmospheric pressure when a substrate on which a film is to be deposited is placed therein or when the substrate is removed therefrom or when one or both of the microwave introducing windows is replaced by a new one; (ii) manual operation is required for replacement or removl of the substrate and because of this, improvements in the quality and the yield of the products can not be anticipated because of unavoidable contamination of foreign matters and so forth; (iii) in order to improve the situation mentioned in the above (ii). the replacement and removal of the microwave introducing window or the substrate are commonly performed under vacuum condition, but it is necessary for the MW PCVD apparatus to be additionally provided with a relevant transportation mechanism for this purpose and such transportation mechanism will be complicated as the number of substrates is increased; and (iv) in the case of the above (iii), as the microwave introducing window(s) and the substrate(s) are transported separately, it takes much time to perform this operation.